Function mx_query_as 

pub unsafe fn mx_query_as<V>(sexp: SEXP, tag: mx_tag) -> Option<&'static V>

Query an object for an interface and return a typed view.

Looks up tag in the object’s vtable registry and, if found, returns Some(&V) pointing to the data pointer + vtable pair for the trait.

Type Parameters

• V - The view type (e.g., FooView) containing data pointer and vtable

Arguments

• sexp - R external pointer wrapping an erased object
• tag - Tag identifying the requested trait interface

Returns

• Some(&V) if the object implements the trait
• None if the object does not implement the trait

Safety

• sexp must be a valid SEXP
• V must be the correct view type for tag
• Must be called on R’s main thread

Example

if let Some(view) = unsafe { mx_query_as::<FooView>(obj, TAG_FOO) } {
    let result = view.some_method(args);
} else {
    panic!("object does not implement Foo");
}